Transparent molding materials resistant to stress cracking and having improved toughness/rigidity ratio

Abstract

Thermoplastic molding materials contain a mixture of (A) a methacrylate polymer, (B) a copolymer of vinylaromatic monomers and vinyl cyanide, (C) a graft copolymer comprising a core and a graft shell and (D) a further graft copolymer comprising a core and a graft shell, the average particle size (d 50 )-C of the graft copolymer (C) being from 25 to 60 nm and that of the graft copolymer (D) being from 90 to 300 nm and the ratio of the average particle size (d 50 )-C to (d 50 )-D being kept in the range from 2:1 to 5:1.

We claim: 
     
       1. A thermoplastic molding material containing a mixture of (A) from 15 to 70% by weight of a methacrylate polymer obtainable by polymerization of a mixture consisting of (A1) from 90 to 100% by weight, based on (A), of methyl methacrylate and   (A2) from 0 to 10% by weight, based on (A), of a C 1  -C 8  -alkyl ester of acrylic acid and     (B) from 10 to 50% by weight of a copolymer obtainable by polymerization of a mixture consisting of (B1) from 78 to 88% by weight, based on (B), of a vinylaromatic aromatic monomer and   (B2) from 12 to 22% by weight, based on (B), of a vinyl cyanide and     (C) from 18 to 35% by weight of a graft copolymer (C) obtainable from (C1) from 50 to 80% by weight of a core obtainable by polymerization of a monomer mixture consisting of (C11) from 50 to 100% by weight of a 1,3-diene and   (C12) from 0 to 50% by weight of a vinylaromatic monomer and     (C2) from 20 to 50% by weight of a graft shell obtainable by polymerization of a monomer mixture in the presence of the core (C1), consisting of (C21) from 40 to 100% by weight of a C 1  -C 8  -alkyl ester of methacrylic or acrylic acid and   (C22) from 0 to 60% by weight of a vinylaromatic monomer,     having an average particle size (d 50 )-C of the graft copolymer (C) of from 25 to 60 nm, and     (D) from 2 to 15% by weight of a graft copolymer (D) obtainable from (D1) from 50 to 80% by weight of a core obtainable by polymerization of a monomer mixture consisting of (D11) from 50 to 100% by weight of a 1,3-diene and   (D12) from 0 to 50% by weight of a vinylaromatic monomer and     (D2) from 20 to 50% by weight of a graft shell comprising (D21) from 40 to 100% by weight of a C 1  -C 8  -alkyl ester of methacrylic or acrylic acid and   (D22) from 0 to 60% by weight of a vinylaromatic monomer,     having an average particle size (d 50 )-D of the graft copolymer (D) of from 90 to 300 nm, and with the further proviso that the ratio of the average particle sizes (d 50 )-C to (d 50 )-D is kept in the range from 2:1 to 5:1, and     (E) optionally, conventional additives E1 in amounts of up to 20% by weight, based on the sum of the components A, B, C and D, the percentages by weight of A, B, C and D summing to 100 and the difference between the refractive index of the components C and D and that of the mixture of the components A, B and, if present, E1 being less than 0.005.   
     
     
       2. A thermoplastic molding material as claimed in claim 1, wherein the graft copolymer (C) has an average particle size (d 50 )-C of from 30 to 55 nm. 
     
     
       3. A thermoplastic molding material as claimed in claim 1, wherein the graft copolymer (D) has an average particle size (d 50 )-D of from 100 to 250 nm. 
     
     
       4. A thermoplastic molding material as claimed in claim 1, wherein the ratio of the average particle sizes (d 50 )-C to (d 50 )-D is kept in the range from 3:1 to 5:1. 
     
     
       5. A process for the preparation of a thermoplastic molding material as claimed in claim 1, wherein (A) from 15 to 70% by weight of a methacrylate polymer obtainable by polymerization of a mixture consisting of (A1) from 90 to 100% by weight, based on (A), of methyl methacrylate and   (A2) from 0 to 10% by weight, based on (A), of a C 1  -C 8  -alkyl ester of acrylic acid and     (B) from 10 to 50% by weight of a copolymer obtainable by polymerization of a mixture consisting of (B1) from 78 to 88% by weight, based on (B), of a vinylaromatic monomer and   (B2) from 12 to 22% by weight, based on (B), of a vinyl cyanide and     (C) from 18 to 35% by weight of a graft copolymer (C) obtainable from (C1) from 50 to 80% by weight of a core obtainable by polymerization of a monomer mixture consisting of (C11) from 50 to 100% by weight of a 1,3-diene and   (C12) from 0 to 50% by weight of a vinylaromatic monomer and     (C2) from 20 to 50% by weight of a graft shell obtainable by polymerization of a monomer mixture in the presence of the core (C1), consisting of (C21) from 40 to 100% by weight of a C 1  -C 8  -alkyl ester of methacrylic or acrylic acid and   (C22) from 0 to 60% by weight of a vinylaromatic monomer,     having an average particle size (d 50 )-C of the graft copolymer (C) of from 25 to 60 nm, and     (D) from 2 to 15% by weight of a graft copolymer (D) obtainable from (D1) from 50 to 80% by weight of a core obtainable by polymerization of a monomer mixture consisting of (D11) from 50 to 100% by weight of a 1,3-diene and   (D12) from 0 to 50% by weight of a vinylaromatic monomer and     (D2) from 20 to 50% by weight of a graft shell comprising (D21) from 40 to 100% by weight of a C 1  -C 8  -alkyl ester of methacrylic or acrylic acid and   (D22) from 0 to 60% by weight of a vinylaromatic monomer,     having an average particle size (d 50 )-D of the graft copolymer (D) of from 90 to 300 nm, and with the further proviso that the ratio of the average particle sizes (d 50 )-C to (d 50 )-D is kept in the range from 2:1 to 5:1, and     (E) optionally, conventional additives E1 in amounts of up to 20% by weight, based on the sum of the components A, B, C and D, the percentages by weight of A, B, C and D summing to 100 and the difference between the refractive index of the components C and D and that of the mixture of the components A, B and, if present, E1 being less than 0.005, are mixed at from 200 to 300° C.   
     
     
       6. Process in which the thermoplastic molding material as claimed in claim 1 is molded. 
     
     
       7. A molding obtainable by the process of claim 6.